This invention relates to a shape memory alloy containing niobium carbide and a process for producing the same. More specifically, the invention relates to a novel shape memory alloy of Fexe2x80x94Mnxe2x80x94Si system that contains niobium carbide and exhibits a sufficiently satisfactory shape memory effect without undergoing training and a process for producing the same.
Considerable attention has been directed to shape memory alloys in the fields of actuator mechanisms, joint mechanisms, and switch mechanisms or as functional materials having shape-restoring properties in a variety of fields. Application of the shape memory alloys to various fields has been proceeding in recent years.
Shape memory alloys having various compositions have been examined so far. Of these alloys, the shape memory alloys of Fexe2x80x94Mnxe2x80x94Si system containing Fe, Mn, and Si as principal constituents (furthermore, including Fexe2x80x94Mnxe2x80x94Sixe2x80x94Cr system and Fexe2x80x94Mnxe2x80x94Sixe2x80x94Crxe2x80x94Ni system) have been developed in Japan.
It is worth notice that the shape memory alloys of Fexe2x80x94Mnxe2x80x94Si system are first discovered in Japan.
However, it is a matter for regret that the alloys of Fexe2x80x94Mnxe2x80x94Si system are not yet put to practical use. The main cause is that the alloys cannot exert a sufficient shape memory effect without undergoing a particular thermomechanical treatment termed training. The training means herein to repeat a heat treatment several times, which consists of 2xe2x80x943% deformation and the subsequent heating above the reverse transformation temperature.
Thus, the shape memory alloys of Fexe2x80x94Mnxe2x80x94Si system in the related art require such troublesome and burdensome training, failing to turn the alloys to practical use.
The invention aims at solving the problem that the shape memory alloys of Fexe2x80x94Mnxe2x80x94Si system in the related art encounters, and providing an novel shape memory alloy of Fexe2x80x94Mnxe2x80x94Si system that exhibits a sufficiently satisfactory shape memory effect without undergoing the special treatment termed training.
In order to solve the aforesaid problems, first, the invention provides a shape memory alloy characterized by containing niobium carbide in the structure in the shape memory alloys of Fexe2x80x94Mnxe2x80x94Si system containing at least Fe, Mn, and Si as principal constituents.
The invention provides, secondly, the aforesaid shape memory alloy containing further Cr or Cr and Ni as principal constituents, thirdly, the shape memory alloy where niobium carbide is contained in volume ratio of 0.1 to 1.5 percent, and fourthly, the shape memory alloy where the alloy composition of niobium and carbon Nb/Cxe2x89xa71 in atomic ratio.
The invention provides, fifthly, a process for producing the shape memory alloy of any one of the aforesaid first to fourth inventions, the process characterized in that an alloy after making an ingot by adding niobium and carbon undergoes a heat treatment for homogenization at a temperature ranging from 1000xc2x0 C. to 1300xc2x0 C. and subsequently, an aging at a temperature ranging from 400xc2x0 C. to 1000xc2x0 C. to precipitate niobium carbide.
The invention has the features as described above, and the embodiments of the invention are described below.
In the shape memory alloys of Fexe2x80x94Mnxe2x80x94Si system containing Fe, Mn, and Si as principal constituents and further Cr or Cr and Ni as needed as principal constituents, the shape memory alloys of the invention are characterized in that niobium carbide is contained in the structure of the alloys. The shape memory alloys of the invention can develop a satisfactory shape memory effect without requiring troublesome, burdensome special treatment termed training in the related art because of the niobium carbide contained in the structure.
Addition of niobium (Nb) and carbon (C) to the structure of the alloy alone cannot develop this effect of the invention. The presence of niobium carbide, that is, the presence thereof as precipitate in the parent phase (austenite) cannot be missed for developing the effect.
The volume ratio of niobium carbide in the crystalline structure desirably ranges from 0.1 to 1.5 percent and more suitably from 0.3 to 1.0 percent.
The volume ratio less than 0.1 percent needs the training in order to expect development of the effect of the invention. On the other hand, exceeding 1.5 percent causes cutting workability to deteriorate; such alloys are unpreferred in view of practical use.
The chemical compositions (weight percent) of the shape memory alloys in general are considered as follows:
 less than Fexe2x80x94Mnxe2x80x94Si greater than 
Mn: 15 to 40
Si: 3 to 15
Fe: the rest
 less than Fexe2x80x94Mnxe2x80x94Sixe2x80x94Cr greater than 
Mn: 5 to 40
Si: 3 to 15
Cr: 1 to 20
Fe: the rest
 less than Fexe2x80x94Mnxe2x80x94Sixe2x80x94Crxe2x80x94Ni greater than 
Mn: 5 to 40
Si: 3 to 15
Cr: 1 to 20
Ni: 0.1 to 20
Fe: the rest,
xe2x80x83and moreover,
Cu: xe2x89xa63 (ppm)
Mo: xe2x89xa62
Al: xe2x89xa610
Co: xe2x89xa630
N: xe2x89xa65000
Of course, unavoidable contamination of impurities is permitted.
The chemical compositions of the shape memory alloys of the invention containing niobium carbide are added with the following composition (weight percent) as a standard:
Nb: 0.1 to 1.5
C: 0.01 to 0.2
In any case, the volume ratio of niobium carbide formed of niobium and carbon preferably ranges from 0.1 to 1.5 percent as described above, and the atomic ratio of niobium to carbon Nb/C is preferably 1 or more and more preferably ranges from 1.0 to 1.2.
The preparation of the shape memory alloys of Fexe2x80x94Mnxe2x80x94Si system that contain niobium carbide as described above is suitably carried out as follows: trace amounts of niobium and carbon are mixed together with specified element raw materials to make an ingot, subjected to a heat treatment for homogenization at a temperature ranging from 1000xc2x0 C. to 1300xc2x0 C. and subsequently, an aging at a temperature ranging from 400xc2x0 C. to 1000xc2x0 C. to allow precipitation of niobium carbide.
More suitably, the heat treatment for homogenization is carried out at a temperature of 1150xc2x0 C. to 1250xc2x0 C. for 5 to 20 hours, and the aging is carried out at a temperature of 700 to 900xc2x0 C. for 0.1 to 5 hours.